Mast cells (MCs) via their secretory products tryptase, histamine and TNFalpha may influence fibrotic processes. This is of interest in the human testis, where infertility occurs in parallel with abundant activated MCs and fibrotic remodelling. We have developed a culture model to study the physiology of fibrotic remodelling using human testicular peritubular cells (HTPCs). Since we found that human testicular MCs are immunoreactive for TNFalpha, we initiated the present study. RT-PCR showed that HTPCs express TNFR1 and TNFR2 and TNFR2 protein was found by immunocytochemistry. Signaling pathways of TNFalpha may include changes in intracellular calcium levels [Ca2+]i, we thus challenged FLUO-4 loaded HTPCs with human recombinant TNFalpha (100 ng/ml). This evoked transient increases of [Ca2+]i in 30 ­40 % of cells. Histamine, via H1 receptors, robustly increases [Ca2+]i in all HTPCs. Interestingly, a preceding challenge with histamine resulted in a sensitization for TNFalpha in those HTPCs, which were insensitive for TNFalpha alone, implying an as yet unknown interaction between these two MCs products. The action of TNFalpha in HTPCs extends to regulation of several genes (e.g., matrixproteins), as revealed by gene array analyses. We thus conclude that TNFalpha plays an important role in regulation of peritubular cell function and infertility in the human testis. (DFG MA 1080/16-1).